The present invention relates to sheet feeding units, such as feed sheets to printing machines or the like, and of the type including a stack elevator operative for raising the top of a stack of sheets to infeed height.
With such sheet feeding units, it is important that the sheets fed into the cooperating machine be fed in without excessive offset in the direction transverse to the infeed direction. However, this condition is not always easy to achieve. For example, if a very tall stack of sheets is laid onto the stack elevator, and its left and right side faces are markedly non-flat, e.g., convex or concave or wavy, it is not simple to force the side faces of so tall a stack of sheets into a flat, and therefore transversely aligned, condition.
German patent DT-PS No. 2,200,755 discloses a system for automatically eliminating transverse offset in the positioning of the sheets of the stack. That system includes two sensors, each located near the top of the stack, but at different respective sides of the stack, i.e., one at the left side of the stack, the other at the right side of the track. Each sensor is operative for sensing the position of the top sheets at its respective side of the stack, and in particular for determining whether the top sheets at its side of the stack are offset in a single respective direction. Each sensor cooperates with a respective switch. Depending upon which of the two switches is activated, a positioning motor shifts the stack elevator in one or the other direction, so as to correct the position of the top sheets.
However, the use of two cooperating sensors, each located at one side of the stack, and each operative for sensing offset in only one direction, brings about considerable practical inconvenience, with respect to servicing, setting-up, adjustment and tolerances. Firstly, the stack, when laid onto the elevator, must already be quite near a condition of no offset; otherwise, when the stack is raised at high speed to operative height, the top face of the stack will hit one or the other sensor from below, and either damage it or render it inoperative for signal generation so long as it rests on the top of the stack. Thus, after the stack has been laid on the elevator, a certain amount of manual work must be done, to bring the stack as much as possible into a correct position of no transverse offset, i.e., precisely that which the automatic offset-correction system should spare the operator from doing. Furthermore, the operator must adjust the settings of the two sensors with an inconvenient amount of carefulness and precision, in dependence upon sheet format (size). If the two sensors are set somewhat too close to each other, the negative-feedback offset-correction system becomes inoperative, because both sensors simultaneously activate their switches, thereby indicating that the top sheets in the stack are simultaneously offset in both the left and right directions, a condition to which the corrective system cannot respond. On the other hand, if the two sensors are set somewhat too far from each other, the negative-feedback corrective system ceases to be useful for its intended purpose.